According to People, Allen and Harbour attended the BAFTA tea party together in January 2019, sparking speculation over the nature of their relationship.

Harbour later revealed to GQ that he met Allen on Raya, an exclusive celebrity-favored dating app, while he was filming Marvel's "Black Widow" in London.

"I started texting with her, she was in Italy at the time — we got together, went on a date at the Wolseley, and it was, you know, she's fucking unbelievable," Harbour told GQ.

He also said he fell in love with her during their third date.

"I was just in this phase, where I was like, I'm just going to be brutally honest about everything, because why lie? And I told her something about my life, about my beliefs," he said. "It would take a really extraordinary person to be accepting of the things that I said. And I remember thinking: Wow, that's somebody I want to be around."

E! News reported that Allen and Harbour saw an evening performance at Piccadilly Theater, and later took a post-show walk and dined together.

Harbour hosted "Saturday Night Live" in October 2019. Shortly after his episode aired, Allen posted a close-up photo of Harbour's muscular arms (taken from a Soul Cycle parody sketch) to her Instagram story, adding the caption, "Mine." 

As People reported at the time, Allen also accompanied Harbour to the "SNL" after-party. The following day, the couple was photographed kissing while walking around New York City.

Harbour and Allen were photographed walking the red carpet at the Skin Cancer Foundation's Champions for Change gala, alongside other celebrities like Sebastian Stan and Katie Holmes. 

The actor shared a mirror selfie with Allen at Disney. In the photo, Allen adjusts her princess costume, while Harbor, who appears to be wearing a cape, smiles at the camera.

"The prince, princess and the parking lot. That seven dwarves mine ride is the best one in the whole damn park. Go on, fight me," Harbour captioned the post on Instagram. 

In a collage of photos the actor shared on Instagram, Allen and her two daughters are shown sitting at a table surrounded by confetti, balloons, and a "Happy Birthday" banner.

"To all the friends and fans that reached out with birthday wishes yesterday, thank you, so happy to have so so so so many people out there tracking my race to being infirm and embarrassingly saggy all over. And to those that didn't, you're heartless jerks," Harbour wrote in his caption.

Allen and Harbour got married in a surprise, low-key Las Vegas ceremony.

Harbour shared several photos from the couple's wedding day, including one of him carrying Allen (clad in a short white dress and black heels) and posing with their officiant, an Elvis impersonator. 

"In a wedding officiated by the king himself, the people's princess wed her devoted, low born, but kind credit card holder in a beautiful ceremony lit by the ashen skies courtesy of a burning state miles away in the midst of a global pandemic," Harbour captioned the post on Instagram. 

He also included a photo of the couple and Allen's daughters eating In-N-Out burgers after the ceremony, which he jokingly described as a "small reception."

During a 2021 appearance on "Jimmy Kimmel Live!," Harbour explained that he and Allen got engaged "right before the pandemic" began in 2020. 

"We were riding bikes, out in the country, trying to figure out what we were going to do for the next year... and the little one was riding along and she was going like, 'David, dad, David, dad.' Because the D got her confused," Harbour explained."And the older one got very upset with that, which I understand. She was like, 'He's not our dad! He's not our dad!'"

"And then the younger one was like, 'Well, what is he? He's kind of our dad.' She's like, 'No, he's not, he's our stepdad.' And she goes, 'What is he?' And she goes, 'He's just some guy in our lives!'" Harbour continued.

Harbour said the conversation between Allen's daughters made him realize he needed to marry her.

Allen, who appears to have wiped her Instagram of all posts before February 2022, reportedly shared several photos with Harbour to commemorate their first anniversary.

According to People, the post included snapshots from their Las Vegas wedding and a video of Harbour dancing.

"1 year down with this guy. Happy Anniversary baby @dkharbour," Allen wrote in the caption. "I prefer my life with you in it. You can stay." 

Allen shares two daughters with her ex-husband, Sam Cooper: Ethel Mary, born in 2011, and Marnie Rose, born in 2013. 

During his relationship with Allen, Harbour shared numerous photos and videos with the girls on social media. The blended family made their first appearance as a unit at the "Stranger Things" premiere in Brooklyn in 2022. 

In a feature for Architectural Digest, Allen and Harbour were photographed in their colorfully decorated home, a late-19th-century Italianate brownstone in the Brooklyn neighborhood Carroll Gardens.

"I tried to reach for something weird and wonderful," said Allen, who worked with designer Billy Cotton and architect Ben Bischoff to transform the interiors. Harbour also praised Allen's "vision."

"My kids call this the clown house, but they say it in the most loving way," Allen added.

According to TMZ, the couple originally purchased the four-story townhouse in 2021 for $3,350,000.

In an interview with The Sunday Times, Allen said she and Harbour were making an effort to limit their screen time.

"I now have a kids' phone called Pinwheel," Allen said. "It has no browsing capability and no social media, but you can still have Uber and Spotify."

"My husband is the caregiver on it, so he controls what I'm allowed to have as an app on my phone," she continued. "I'm the controller of his as well. Because they're made for kids, he's my parent and I'm his parent."

During an episode of her BBC Sounds podcast "Miss Me?," Allen discussed sexual preferences with her cohost and friend, Miquita Oliver.

"I wonder if I kink-shame my husband," she said. "Because he often quite asks for things, and I'm like, 'No, babe, it's not happening.'"

"I'm not like, 'You piece of shit, how dare you ask me to do that!'" Allen added. "I'm just like, 'Hmm, headache. Little headache, got a bit of a headache — maybe not tonight.'"

During another episode of "Miss Me?," Allen opened up about her recent decision to sell photos of her feet on the adult-content platform OnlyFans. (Allen said the photos were more lucrative than streaming payouts from Spotify.)

Asked how Harbour felt about the venture, Allen said, "He thinks it's great."

"At first, he was like, not turned on," she continued, "but he was like, 'Is this a kink for you?' And I was like, 'No, it's totally not a kink,' but maybe there's something in the power element of it that's slightly kinky for me."

Allen opened up about her disordered eating habits during an episode of "Miss Me?"

"I'm really not in a great place mentally at the moment," Allen told her cohost. "My body and my brain are so disconnected from each other that the messages of hunger are not going from my body to my brain."

Allen later revealed that she wrote her breakup album, "West End Girl," that same month during a 10-day creative burst.

"Nobody knew what was going on in my life," Allen told The Sunday Times. "So I got into the studio, cried for two hours, and then said, 'Let's make some music.'"

On February 3, 2025, People reported that Allen and Harbour had separated.

"Her marriage has been crumbling, and they have split," an anonymous source told the publication. Representatives for Allen and Harbour did not respond to requests for comment from People.

Shortly after, Allen revealed during an episode of "Miss Me?" that she'd recently spent a few weeks in a treatment center, receiving therapy for "emotional turmoil."

"I needed some time and space away from everything. I did a lot of shadow work, lots of work about my inner child stuff," she said.

"I'm not saying I'm 100% there," she continued. "I'm not saying that I'm getting it 100% right or ever will. But I'm definitely in a stronger place."

During an interview with GQ, Harbour refused to confirm or deny reports of his marriage ending.

"I'm protective of the people and the reality of my life," he said. "There's no use in that form of engaging [with tabloid news] because it's all based on hysterical hyperbole."

In "West End Girl," Allen tells a brutal story of marital turmoil and betrayal, portraying her muse as an emotionally manipulative sex addict.

"Who said romance isn't dead? Been no romance since we wed," she sings in track three, "Sleepwalking." In several other songs, including "Ruminating," "Tennis," and "4chan Stan," she alludes to catching her husband being intimate with other women, crossing boundaries they'd agreed upon.

"We had an arrangement / Be discreet and don't be blatant / There had to be payment / It had to be with strangers / But you're not a stranger," she sings in track five, "Madeline." Two songs later, Allen seems to accuse Harbour of keeping a separate apartment in the West Village expressly for affairs, and describes finding sex toys and "hundreds" of condoms hidden there.

In interviews, Allen has described the album as semi-autobiographical. (She has also begun referring to Harbour as her ex-husband, per British Vogue, though no divorce filings have yet been made public.)

In the opening track on "West End Girl," Allen recalls house-hunting in New York and buying an expensive brownstone with Harbour.

"Billy Cotton got sorted / All the furniture ordered," she sings, name-checking the designer whom she hired to help decorate. "I could never afford this / You were pushing it forward / Made me feel a bit awkward."

Just a few days after Allen's album was released, the former couple's Brooklyn home was put back on the market.

TMZ reported that the five-bedroom, four-bath brownstone was listed for just under $8 million.